In a construction vehicle such as a hydraulic power shovel and crane, generally various operations are carried out through remote control of various actuators by means of a hydraulically operated pilot type control valve by an operator mounted on the vehicle. Various actuators and operating devices provided in the construction vehicle are large in size and heavy, and cause vehicle malfunctions when the operator makes an abrupt operation, making it unable to operate the vehicle normally due to substantial rolling and vibration of the vehicle body. Also rolling and vibration of the construction vehicle caused by the travel or intended operation thereof are transmitted, whether or not via the operator's hand and foot, to the hydraulic control valve causing undesirable fluctuation in the amount of operation of the hydraulic valve so that, as a result, the rolling and vibration of the construction vehicle are aggravated. Therefore, fluctuation in the amount of operation of the hydraulic valve must be made as small as possible, either when the operator operates to pivot the operating member of the hydraulically operated valve off the neutral position thereof, or when the operator operates the operating member to return it from the pivoted position to the neutral position. For this reason, hydraulically operated valves equipped with damping means have been proposed.
FIG. 5 shows a cross sectional view of a typical prior art hydraulically operated valve. This prior art hydraulic valve is disclosed in Japanese Patent Application Laid-Open Sho No. 61-294281. In this prior art valve, a pivoting member 1 is made to pivot in a pivoting direction A1 by the operation of an operating member such as a pedal or lever, thereby to engage and depress an end of a push rod 2. The push rod 2 is inserted through a damper chamber 4 formed in a casing 3, whereas a piston 2p is formed in an intermediate section and a sealing section 2s of the casing 3 is inserted through the upper portion thereof. The piston 2p separates the damper chamber 4 into an upper chamber 4a and a lower chamber 4b. The upper chamber 4a and the lower chamber 4b communicate with each other via damping means 5. The damping means 5 includes a throttle 5a and a check valve 5b. The lower part of the push rod 2 is fitted into a spring chamber 6 that is formed in the casing 3. Disposed in the spring chamber 6 are a return spring 6a that presses the push rod 2 upward and a pressure spring 6b that transmits the pressing force applied from the bottom face of the push rod 2 to a spool valve 7. A top portion 7c of the spool valve 7 is fitted into a recess 2a formed at the bottom of the push rod 2. The spool valve 7 has an oil passage 7a formed at right angles to the axis thereof and an oil passage 7b extending in the axial direction from the oil passage 7a to the bottom face of the spool valve 7, both being formed thereon. The casing 3 has a pump port 8p, an output port 8o and a tank port 8t installed therein. At the neutral position shown in FIG. 5, the output port 8o and the tank port 8t are in fluid communication with each other through the spool valve 7 and the pump port 8p is shut off.
When the pivoting member 1 is pivoted in the pivoting direction A1, the push rod 2 is pressed downward so that the spool valve 7 is pressed downward via the pressure spring 6b. As the oil passage 7a perpendicular to the axis fluidly communicates with the pump port 8p which is located below the tank port 8t, the tank port 8t is shut off while the pump port 8p and the output port 8o fluidly communicate with each other. At this time, because the hydraulic oil in the damper chamber 4 moves from the lower chamber 4b to the upper chamber 4a through the throttle 5a, a damping effect can be obtained. When the pivoting member 1 returns in a direction A2 which is reverse to the pivoting direction A1, the push rod 2 is forced to move upward by the return spring 6a so that the hydraulic oil in the damper chamber 4 moves from the upper chamber 4a through the throttle 5a and the check valve 5b into the lower chamber 4b. At this time, because the check valve 5b opens, resistance against the hydraulic oil flow is reduced and therefore no damping effect can be obtained.
In such a prior art hydraulic valve as described above, while a damping effect is obtained when the pivoting member 1 pivots from the neutral position in the arrow A1 direction, there is a problem in that a damping effect cannot be obtained when the pivoting member 1 returns from the pivoted position in the arrow A2 direction to the neutral position because the check valve 5b is opened at this time.
FIG. 6 shows a cross sectional view of another prior art hydraulically operated valve. This prior art hydraulic valve is disclosed in Japanese Utility Model Laid-Open No. Hei 4-93501. In this prior art hydraulic valve, a cam member 11 is forced by the operation of an operating member such as a pedal to pivot in two directions indicated by arrows B1 and B2 around an axis of a shaft 11a thereby to press either one of a pair of spool valves (made in integral bodies with push rods 10, 10'), not shown in the drawing, via pistons 19 or 19', auxiliary push rods 12 or 12' and push rods 10 or 10' while another one is returned. On both sides of the shaft 11a of the cam member 11, bolts 11b, 11b' are fixed by means of nuts 11c, 11c', respectively. Heads of the bolts 11b, 11b' make contact with the pistons 19, 19', each piston having a cross section of inverted U shape. The pistons 19, 19' have oil chambers 19a, 19a' opening downward formed therein. Hydraulic oil contained in spring chambers 16, 16' under a drain pressure is introduced into the oil chambers 19a, 19a' via grooves 19b, 19b' holes 19c, 19c' recesses 19d, 19d' and holes 19e, 19e'.
A casing 13 is fixed by means of a bracket 13f onto a floor surface or the like of an operator's cabin wherein the hydraulic operated valve is installed. Installed in the damper chambers 14, 14' are damping springs 14s, 14s'. Installed at the bottom of cylinders 13p, 13p' are throttles 15a, 15a' and check valves 15b, 15b', to facilitate fluid communication between the damper chambers 14, 14' and the spring chambers 16, 16'. Installed in the spring chambers 16, 16' are return springs 16a, 16a' that press spring receiving pieces 17, 17' upward and pressure springs 16b, 16b' that press the spool valves (not shown in the drawing) downward during operation, with the top ends of the springs 16a, 16a', 16b, 16b' being in contact with the spring receiving pieces 17, 17'. Penetrating through central portions of the spring receiving pieces 17, 17' are push rods 10, 10' with heads 101a, 101a' thereof being fitted in recesses 19d, 19d' of the auxiliary push rods 12, 12'. A bellows 20 is installed across the cam member 11 and the bracket 13f, thereby to protect the inner mechanism of the hydraulic valve.
When the operator operates the operating member to cause the cam member 11 to pivot off the neutral position in the arrow B1 direction, one bolt 11b on the pivoting direction B1 side forces the piston 19 downward with the auxiliary push rod 12 that is in contact with the piston 19 being pressed downward at the same time, so that the spring receiving piece 17 is displaced downward and the pressure spring 16b presses one spool valve downward. At this time, hydraulic oil in the damper chamber 14 is made to flow via the throttle 15a and the groove 19b into the spring chamber 16 by the downward movement of the piston 19. Resistance against the hydraulic oil flow is generated by the throttle 15a as described above, making it possible to obtain a damping effect during pivoting from the neutral position to the arrow B1 direction.
On the other hand, the pivoting movement of the cam member 11 in the arrow B1 direction causes another bolt 11b' to move upward so that the piston 19' is forced to move upward by the damping spring 14s' and, as the inner volume of the damper chamber 14' increases, the hydraulic oil flows from the spring chamber 16 through the check valve 15b' to supplement the pressure in the damper chamber 14' while the oil chamber 19a' in the piston 19' communicates with the spring chamber 16 through the groove 19b', the hole 19c', the recess 19d' and the hole 19e', thereby to be filled with the hydraulic fluid under the drain pressure. Thus the piston 19' is always kept in contact with the bolt 11b'.
When the operator releases the operating member such as a pedal from the pressed state under such a condition as described above, the state of pressing the cam member 11 off the neutral position in the arrow B1 direction is canceled so that the spring receiving piece 17 which has been pressed downward by the auxiliary push rod 12 moves upward by the action of the return spring 16a. Thus the pressure spring 16b is released from the compressed state to cause one of the spool valves (not shown in the drawing) to move upward and return to the neutral position. At this time, the cam member 11 pivots in the arrow B2 direction toward the neutral position and the piston 19' is pressed downward by the bolt 11b', so that the hydraulic oil in the damper chamber 14' is forced back through the throttle 15a' into the spring chamber 16.
When the cam member 11 returns from the state of being pivoted in the arrow B1 direction to the neutral position, as described above, resistant force is exerted by the throttle 15a' against the hydraulic oil which flows from the damper chamber 14' into the spring chamber 16 even when an external force is exerted on the cam member 11 in the arrow B2 direction, for example, due to rolling and vibration of the vehicle body, and consequently an abrupt pressure variation is generated in the damper chamber 14' making the piston 19' unable to move quickly. This makes it possible to obtain damping effect when the operator returns the operating member to the neutral position. Also when the operating member is pivoted from the neutral position in the arrow B2 direction, or when the operating member is returned from the pivoted position to the neutral position, a damping effect can be obtained similarly, thus solving the problems of the prior art hydraulic valve shown in FIG. 5.
In such a prior art hydraulic valve as shown in FIG. 6, bubbles accumulate in the damper chambers 14, 14' and cannot be easily purged to the outside environment. Because such bubbles reduce the damping effect significantly, they should be purged at an early stage. Also it is necessary to contain a high pressure generated in the damper chambers 14, 14' during damping by means of the sealing members 10a, 10a', although it is difficult to achieve high reliability because the pistons 19, 19' have large diameters and undergo a sliding motion. Further, the pistons 19, 19', the auxiliary push rods 12, 12' and the cylinders 13p, 13p' have complicated structures requiring high production costs.
In consideration of the problems in the prior art described above, it is desired to provide an improved hydraulically operated control valve of high reliability and lower cost that solves the problems described above, wherein a damping effect can be obtained both when an operating member is moved from a neutral position to a pivoted position and when the operating member is returned from the pivoted position to the neutral position.